The basic components of a battery are the electrodes with terminals (electric connections) to connect to an external circuit, a separator to keep the electrodes apart and prevent them from shorting, the electrolyte which carries the charged ions resulting from the chemical reactions taking place at the electrodes and a cover to contain the active chemicals and hold the electrodes in place.
“Wet” cells refer to galvanic cells where the electrolyte is in liquid form and is allowed to flow freely within the cell casing. “Dry” cells are cells that use a solid or powdery electrolyte. Cells with liquid electrolyte can be classified as “dry” if the electrolyte is immobilized by some mechanism, such as by gelling it or by holding it in place with an absorbent substance such as paper.
The most common type of battery used today is the “dry cell” battery used in e.g. relatively large batteries such as “flashlight” batteries and in miniaturized versions used for wristwatches or calculators.
Batteries are often classified by the type of electrolyte used in their construction. There are three common classifications; acid, mildly acid, and alkaline.
All batteries utilize similar procedures to create electricity; however, variations in materials and construction have produced different types of batteries.
One battery type consists of a layered structure, i.e. those called thin film batteries.
Thin film batteries, which term in this text is to be understood as “layered-structured batteries” regardless of size, can be applied directly onto film applications in any shape or size. Flexible thin batteries can be made by printing on to paper, plastics, or other kind of thin foil and they are also called printed batteries.
Thin film batteries have e.g. a wide range of uses as power sources for consumer products and for micro-sized applications. Thin film batteries are also suitable for powering smart cards and Radio Frequency IDentification (RFID) tags.
The method of manufacturing such a thin battery can be done by wetting the separator paper with an electrolyte solution and applying an anode material and a cathode material as pastes on the separator paper(s), the anode paste on one side and the cathode paste on the opposite side. The anode and cathode materials can be applied on the separator papers with different methods such as by coating or printing. The coating and printing processes generally involve the application of a thin film of functional material to a substrate, such as roll of paper, fabric, film or other textile.
With the term paste, it is in this text just meant, a viscous water-based dispersion of particles.
However, also the outmost separator layers will always contain electrolyte, since they absorb electrolyte from the wetted layer and furthermore, the electrolyte solution, that contains additives, might also be mixed with the anode active material and cathode active materials to form so called anode and cathode pastes.
The electrodes of a thin battery are formed of the anode and the cathode. The anode material is a paste or ink containing an anode active material and usually electrolyte solution with additives and the cathode material is a paste or ink containing a cathode active material and usually electrolyte solution with additives. The paste is quite viscous so that the electrolyte is not capable of flowing out. The anode active material is often zinc (Zn) and the cathode active material is manganese dioxide (MnO2). The electrolyte solution can e.g. contain ZnCl2 as a main ingredient as well as additives, such as binder(s). The additive(s) in the electrolyte comprises binder(s) in order to provide other properties to the electrolyte solution for example binding the electrode material particles together to form a paste. The binder is e.g. polyvinylalcohol (PVA).
Conductive material is added to the anode and cathode pastes. The conductive material can be carbon powder, such as graphite powder, soot, carbon black, carbon nanotube, conductive ink or combinations thereof. The electrodes (consisting of the anode and cathode pastes inclusive the conductive material) are connected to a collector material and the whole product is covered by films of sealing material. The sealing material can be of e.g. polypropylene, polyethylene, polyester or other known film materials. The collector material is formed to have terminals to be connected to an external circuit. The collector material can be conductive carbon ink, carbon film or other material, which is chemically inert but conductive enough for the purpose.
The combined layers are cut in desired sizes to form a product.
Different formats of anode materials have been tested to facilitate the manufacturing of a thin battery.
US patent application 2006/0216586 is presented as prior art. It discloses a thin electrochemical cell. The anode of the cell can be a zinc strip. The problem with this solution is, however, that the connection of the zinc strip to the collector is problematic and that the zinc self discharge is high because of lacking coverage.
U.S. Pat. No. 6,379,835 is concerned with a thin film battery. The anode is hydrophilic being a water based zinc ink or a zinc ink in an organic solvent+water swellable polymer mixture and therefore the battery has high self discharge rate. Furthermore because of the low conductivity of anode, an extra collector at the anode is needed.
US patent application 2006/0115717 presents a flexible thin printed battery. A conductive aqueous zinc material to constitute the anode is printed directly onto a non-conductive substrate and is made sufficiently conductive to eliminate the need for a distinct anode current collector. The necessary conductivity is achieved by excess zinc+2 cations (from zinc acetate) and polymeric binder (polyvinylpyrrolidone) to improve particle contact. However, also here, Hydrophilic zinc anode causes high zinc self discharge and unstable conductivity during the discharging process.
Further prior art solutions are presented in e.g. WO publication 03/100893 and US006045942.
In addition to the self discharge problem the prior art thin batteries are complicated to manufacture or integrate into application as one of the terminals to be added have to be put on one side and the other one on the other side of the battery. As the terminals are located on the two opposite sides of the thin battery, also the integration with application is complicated. It is difficult to connect the negative and positive electrodes of a device consuming the power to the thin battery as most devices have the electrodes only on one side.
Intensive work is currently done for making constantly improved batteries by solving some of the acknowledged problems in connection with the manufacturing of the batteries and in order to decrease the self discharge.
Another problem in traditional manufacturing methods to overcome is that the paper sheets become quite wet when they are impregnated with the electrolyte and also the paste remains wet after it is applied. This situation influences on the surface properties of the layers so that further manufacturing steps will be difficult to carry out and makes the total manufacturing process of battery difficult.
There is electrolyte in the anode and cathode pastes, too. As is well known, the zinc is quite easily oxidized under wet environment containing electrolyte salt and self-discharge is a problem. Furthermore, the printing or coating of especially the anode paste is also difficult due to the fast changes of the properties of the paste.
The object of the invention is therefore to develop a battery, wherein the above problems are avoided.